Douglas Isbell
Headquarters, Washington, DC December 1, 1995
(Phone: 202/358-1753)
Franklin O'Donnell
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)
RELEASE: 95-215
GALILEO CROSSES BOUNDARY INTO JUPITER'S ENVIRONMENT
NASA's Galileo spacecraft radioed confirmation late
this week that it has entered Jupiter's environment,
crossing over the boundary from interplanetary space into
the giant magnetic cocoon around Jupiter called the
magnetosphere.
"With the spacecraft now in the magnetosphere, we begin
our first direct measurements of the Jupiter system," said
Galileo Project Manager
William J. O'Neil at NASA's Jet Propulsion Laboratory (JPL),
Pasadena, CA.
Data from Galileo's magnetometer confirmed that the
spacecraft passed the milestone on Nov. 26 at a distance of
about six million miles (nine million kilometers) from
Jupiter's cloud tops, scientists announced today.
After a six-year voyage through the Solar System,
Galileo is less than a week away from taking up permanent
residence around Jupiter. On Thursday, Dec. 7, Galileo's
previously deployed atmospheric probe will plunge into
Jupiter's cloud tops at 5:56 p.m. EST and descend into the
giant planet on a parachute.
Overhead, the Galileo spacecraft itself will collect
and record data radioed from the probe during the 40- to 75-
minute probe mission. At 8:19 p.m. EST, an hour after the
probe mission is completed, Galileo will begin to fire its
onboard rocket to slow down and allow itself to be captured
into orbit around Jupiter to begin a two-year mission of
closeup studies of Jupiter's large moons, the planet itself,
and continuous measurements of the magnetosphere.
Jupiter's magnetosphere is like a giant bubble around
the planet. A shock wave -- called "bowshock" after the
wave that builds before the bow of a ship -- exists where
the magnetosphere faces the stream of charged particles
flowing outward from the Sun, called the solar wind. As the
solar wind flows around Jupiter, the magnetosphere tapers
off like a wind sock, with the whole invisible structure
moving in response to buffeting by the solar wind.
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Galileo scientists said they first saw signs of the
bowshock on Nov. 16, but the bowshock apparently moved back
and forth in response to alternate gusts and waning of the
solar wind. "As the solar wind velocity increased, the
shock moved inside the position of the spacecraft leaving
Galileo again in the solar wind," said Dr. Margaret Galland
Kivelson of the University of California at Los Angeles, the
principal investigator on Galileo's magnetometer experiment.
This crossing and recrossing of the shockwave happened
several times, she said, between the first shock encounter
on Nov. 16, when the spacecraft was about nine million
miles (15 million kilometers) from Jupiter, and Nov. 26 when
Galileo finally crossed the main bowshock at 1 p.m. EST at
about six million miles out from Jupiter's cloud tops.
The magnetometer science team also found the first
direct evidence that the jovian magnetosphere was either
unaffected or had recovered in the aftermath of last year's
impact of Comet Shoemaker-Levy with Jupiter. Some
scientists had theorized that the magnetosphere might have
been modified signficantly by the violent impact, but that
appears not to be the case, according to data from Galileo.
Meanwhile, Galileo engineers report that work has been
completed on the spacecraft's tape recorder to assure its
readiness for recording data during Thursday's atmospheric
probe descent. Final fine-tuning of the spacecraft's flight
path is scheduled for this Saturday, Dec. 2.
Two Internet home pages exist to provide information on
the atmospheric probe, Galileo orbiter spacecraft, mission
operations and science returns. The Galileo Project home
page may be accessed at http://www.jpl.nasa.gov/galileo. A
home page sponsored by the atmospheric probe team at NASA
Ames Research Center, Mountain View, CA, may be accessed at
http://ccf.arc.nasa.gov/galileo_probe/.
NOTE TO EDITORS: A press briefing prior to the probe
mission and Galileo's Jupiter orbit-entry will be held at
JPL on Dec. 7 at 4 p.m. EST, and a follow-up briefing will
be held at 9:45 p.m. EST. Both events will be carried live
on NASA Television.
NASA Television is available through the Spacenet 2
satellite, transponder 5, channel 9, 69 degrees West
longitude, frequency 3880 MHz, audio subcarrier 6.8 MHz,
horizontal polarization.
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